Flexible duct with shrinkage-proof film

ABSTRACT

The invention concerns a flexible duct ( 1 ) comprising from inside outwards a carcass ( 2 ) produced by a short-pitched metallic helical winding forming butt gaps ( 12 ), an overlapping layer ( 10 ) acting as a heat shield consisting of helically wound plastic strips ( 10   a   , 10   b ), a sealed inner pressure sheath ( 3 ), extruded in plastic on said overlapping layer ( 10 ), at least an armouring web ( 5 ) and at least an outer sealing sheath ( 7 ). The invention is characterised in that the overlapping strips ( 10   a   , 10   b ) are wound at a winding angle less than 35°.

BACKGROUND OF THE INVENTION

The present invention relates to a flexible pipe that can be used fortransporting fluids, such as hydrocarbons for example.

Several types of flexible pipe are used. Some flexible pipes comprise,from the inside outward, an internal sealing sheath made of a plastic,an elastomer or another relatively flexible suitable material; anunsealed flexible metal tube that has to withstand the forces developedby the pressure of the fluid flowing in the pipe; one or more armorplies and at least one external sealing sheath made of a polymericmaterial. This type of flexible pipe is often called a smooth-bore pipeby experts in the field.

Other flexible pipes, called rough-bore pipes, comprise, from the insideoutward, an unsealed flexible metal tube, called a carcass, formed by aprofile wound in turns and mutually interlocked, such as, for example,an interlocked strip or an interlocked shaped wire such as a T-shaped,U-shaped, S-shaped or zeta-shaped wire; an internal sealing sheath madeof a polymeric material; one or more armor plies capable of withstandingthe forces developed by the pressure of the fluid flowing in the pipeand the external forces to which the flexible pipe is subjected; and atleast one external protected sheath of the polymeric type.

In the latter type of flexible pipe, the internal sealing sheath isextruded, continuously, directly over said carcass, which hasinterstices or gaps between the wound turns.

To ensure good contact between the internal sealing sheath and the metalcarcass, it is necessary for the inside diameter of the internal sealingsheath to be as close as possible and even equal to the outside diameterof the flexible metal carcass.

During manufacture of a rough-bore flexible pipe, the internal sealingsheath, which is extruded over the metal carcass, contracts onto thelatter during cooling. Depending on the materials used for producing theinternal sealing sheath, after cooling deformations called “shrinkagecavities” are observed, these cavities appearing on the internal face ofsaid internal sealing sheath and especially on either side of the gapsbetween the turns of the metal carcass. Such shrinkage cavities are due,it would seem, to the differential shrinkage of the material used forthe internal sealing sheath, because of the variation in the coolinggradient through the thickness of the internal sealing sheath, combinedwith the effect of the gaps between the turns of the metal carcass.Since the extruded plastic sealing sheath is in contact by its internalface with the metal carcass, which is at room temperature, this resultsin said internal face cooling very rapidly, thereby causing surfaceirregularities or shrinkage cavities; this phenomenon is exacerbated atthe gaps between the turns of the metal carcass, the differentialshrinkage at these points causing local variations in the thickness ofthe internal sealing sheath. When the sealing sheath is made of asemicrystalline polymer sensitive to the presence of surface defectscausing a weakening of the sheath, possibly to the point of failure,such as PVDF (polyvinylidine fluoride) for example, this very oftenleads, in operation, to degradation (failure) of said sealing sheath,which then no longer fulfils its sealing function.

To remedy such a drawback and to solve the problem posed by theappearance of shrinkage cavities, a first solution consisted in placing,between the metal carcass and the internal sealing sheath, a thinsacrificial underlayer (thickness about 2 to 3 mm) made of a suitablematerial such as PVDF, which then serves as a heat shield. The internalsealing sheath is extruded over said sacrificial underlayer, but withoutany assurance that there is intimate bonding or “welding” between thesealing sheath and the sacrificial underlayer, so that cracks, that canpropagate from the internal face of the underlayer to the outside, areblocked at the sealing sheath/sacrificial underlayer interface.

The major drawback of this solution is the slip that is liable to occurbetween the internal sealing sheath and the sacrificial underlayer atthe ends of the flexible pipe, and the additional raw material andconversion costs incurred by the presence of said sacrificialunderlayer.

Provision could be made to extrude a thinner sacrificial sheath(thickness less than or equal to 1 mm), but, because of the diameter ofthe extruded tube (greater than 10 cm), it is impossible for so thin atube to be extruded on an industrial scale. It is therefore limited to a2 to 3 mm thick sheath. In addition, the operation requires theintermediate sheath to be wound on an intermediate reel and, since theintermediate sheath is thin, it will buckle during winding.

To avoid these drawbacks, document FR 2 752 904 (COFLEXIP) proposed aprocess for manufacturing flexible pipes that consists in heating theflexible metal tube or metal carcass to a temperature of below 100° C.,upstream of the extrusion means, so as in this way to avoid suddenlycooling the internal face during extrusion over the metal carcass.

For plastics of very low viscosity, it is necessary to heat the productto a very high temperature, and consequently to heat the carcass to ahigh temperature. This high temperature induces very substantial thermalcreep in the gaps of the carcass, requiring the insertion, into thesegaps, of a rod that limits the volume of creep in order to preventblockage of the carcass. Such a rod is described in document FR 2 779797 (COFLEXIP), but the spiraling of the rod in the gaps of the carcassis not simple to implement. Documents EP 0 749 546 (COFLEXIP-ELFATOCHEM) and FR 2 732 441 (COFLEXIP) disclose the short-pitch helicalwinding of an intermediate strip for following and partially filling thegaps, and are therefore similar to the previous solution.

Another solution, proposed in document EP 166 385 (FURUKAWA), consistsin winding, around the carcass, several layers of thin plastic (forexample polyester) tapes (thickness about 0.5 mm for a tube about 2 to 8cm inside diameter). This interlayer masks the gaps and prevents thesealing sheath from creeping into the gaps in the carcass. The detailsof the winding are not explained in the document.

The interlayer, by preventing the sealing sheath from creeping into thecarcass, consequently also prevents the sheath from bonding to thecarcass and therefore creates problems of slip between the two layers.For riser applications, this may cause the flexible pipe to deteriorate:since the gap between the turns is not controlled by the creepindentations, the carcass can slip under its own weight, the gapsbetween turns being canceled out and accumulating at the base of theriser, causing destruction of the carcass in the upper part.

SUMMARY OF THE INVENTION

The objective of the invention is to propose, within the context of aflexible pipe, a carcass-covering system that serves as a heat shieldfor the extruded plastic sheath so as to prevent the formation ofshrinkage cavities, but does not have the drawbacks of theaforementioned solutions and enables it to be put into place simply.

The invention achieves its objective by means of a flexible pipe of thetype comprising, from the inside outward, a carcass formed by ashort-pitch helical metal winding leaving gaps, a covering layer servingas a heat shield, consisting of helically wound plastic tapes, aninternal sealed pressure sheath made of a plastic extruded over saidcovering layer, at least one armor ply wound around said sealing sheathand at least one external sealing sheath, characterized in that thecovering tapes are wound with a long pitch, that is to say with awinding angle of less than 35°.

To do this, a taping unit is placed upstream of the sheath extruder inorder to wind several thin plastic tapes around the carcass with a longpitch, preferably between 10° and 35°.

These tapes are preferably wound with an overlap, thereby making itpossible to ensure that the entire carcass is covered with this layer.This overlapping of the tapes may create a defect on the internalsurface of the extruded sheath, but this possible defect is a long-pitchhelical defect, and therefore does not have a great influence on thesealing sheath. The overlap of the tapes is typically about 10% of thetape width.

By winding the tapes with a long pitch, the scrapers of the extruder areprevented from lifting these tapes up and the length of tapes needed forcovering the entire length of the flexible pipe is thus limited, whichavoids having to recharge the taping unit during production.

The tape is thick enough to serve as a heat shield for the sealingsheath during extrusion (until the internal surface of the sheath hassolidified) in order to prevent the formation of shrinkage cavities, butthin enough, however, to allow indentation of the sealing sheath intothe gaps in the carcass in order to allow bonding thereto.Advantageously, the indentations form approximately 20% to 75% of thevolume of the gap. The thickness of the tape also depends on thematerial of which it is made, but it is generally less than one fifth ofthe thickness of the pressure sheath and preferably about 1 mm or evenless.

Since the tape plays no part in the behavior of the flexible pipe, itmay very well degrade on contact with the extruded plastic sheath, fromthe moment that it nevertheless acts as a heat shield until the internalsurface of the plastic sheath has solidified. It therefore has to have alifetime of approximately one minute.

To be able to install this tape in great length and to avoid any risk offracture during its installation (which would require restarting theextrusion) or any risk of uncontrolled elongation of the tape, it ispreferable for the tape to have a tensile strength of greater than 20daN (the tensile force imposed by the taping unit). This strength may beprovided by the basic constituent material of the tape or byreinforcements, especially by metal reinforcements or textilereinforcements (carbon or Kevlar®-type fibers or rovings) for example inthe form of longitudinal reinforcements or meshes.

The width of the tapes is preferably close to the outside diameter ofthe carcass so as to ensure that it is covered with 3 to 5 tapes.

The constituent material of the covering tapes is chosen to have arelatively low flexural strength (elastic modulus) so as to deform atthe gaps, in order to allow the desired creep when the sheath contracts.

The constituent material of the tapes may be of the reinforced orunreinforced heat-shrinkable type, which may improve the formation ofthe indentations.

The tape may have longitudinal notches (which reduce the tensilestrength of the tape), the indentation of the pressure sheath into thecarcass taking place through these notches.

The tapes must be made of a material compatible with the pressuresheath, that is to say with polyolefins, polyamides (preferably of theRILSAN® is nylon-11 type), modified or unmodified fluoropolymers(homopolymers or copolymers) (polyvinylidene fluoride PVDF orpolyfluoroalcoxy) or hydrocarbon, fluorinated or fluorosiliconelastomers (thermoplastic elastomer or thermoplastic urethane).

The tape must have a flexural strength less than the force ofcontraction of the pressure sheath as it cools down. The flexuralstrength of the tape depends on the hot modulus and the thickness of thetape. The force generated by the contraction of the pressure sheathdepends on the thickness of the pressure sheath, the hot elastic modulusof this sheath and the size of the gap. Those skilled in the art willknown how to determine the behavior of the pressure sheath as it coolsdown, for example using ABAQUS software, and consequently the maximumtape thickness for allowing indentation of the pressure sheath into thecarcass.

In the case in which the tape is eliminated on contact with theextrudate (pressure sheath), an example of a suitable material for thetape is a polyester polyurethane (TPU) known by the name ESTANE® 58271,the viscosity of which decreases rapidly at high temperature (the layermay be eliminated in 30 days at 120°). A small thickness in contact withthe extrudate (at an extrusion temperature above 230° C. in the case ofa PVDF), the tape will have a lifetime of a few minutes, sufficient toallow the internal surface of the pressure sheath to solidify(solidification temperature close to 160-170° C. in the case of a PVDF).

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages and features of the present invention will become moreclearly apparent on reading the following description, together with theappended drawings, in which;

FIG. 1 is a partial perspective view of a rough-bore flexible pipe thatincludes the covering tapes according to the invention;

FIG. 2 is a partial longitudinal sectional view of part of the flexiblepipe of FIG. 1, showing the covering tapes and the sheath that coversthem; and

FIGS. 3 to 5 are schematic representations of tapes suitable for theinvention.

DESCRIPTION OF PREFERRED EMBODIMENTS

A rough-bore flexible pipe 1 comprises, from the inside outward:

-   -   a flexible metal tube or carcass 2, formed by a short-pitch        helical winding (that is to say one with a winding angle close        to 90°) of a metal strip or wire of predetermined cross section;    -   an internal sealing or pressure sheath 3 made of a polymer,        placed around the carcass 2;    -   a pressure vault 4 resistant mainly to the pressure developed by        the fluid in the sealing sheath and formed by the short-pitch        helical winding (that is to say one with a winding angle close        to 90°) around the internal sheath of one or more interlocked        profiled metal wires (which may or may not be        self-interlockable); the profiled wires have a Z-shaped or        T-shaped cross section or one derived therefrom (teta or zeta),        or a U-shaped or I-shaped cross section;    -   one or more armour plies 5 wound with a long pitch, for example        two crossed tensile armour plies whose lay angle, measured along        the longitudinal axis of the pipe, is less than 60°;    -   optionally, one or more intermediate bands 6; and    -   an external sealing sheath 7 made of a polymer.

According to the invention, a covering layer 10, formed by coveringtapes 10 a, 10 b, etc., wound with a long pitch and with a slightmarginal overlap 11, was placed between the carcass 2 and pressuresheath 3. As shown in FIG. 2, at a gap 12 between two adjacent turns ofthe carcass 2 (formed here by an S-shaped self-interlocked strip), afterthe sheath 3 has been extruded, the tape 10 a or 10 b is forced into thegap, forming a helical groove filled by the extruded sheath 3 and thusforming a helical indentation 13.

The tapes forming the covering layer are wound with a long pitch, with awinding angle of less than 35°. According to an extreme case (notshown), the covering layer may be formed by one or more tapes placedlongitudinally along the axis of the pipe (the “winding” angle thenbeing 0°).

FIGS. 3 to 5 show various examples of covering tapes that can be used inthe invention. The tape 10 a in FIG. 3 has longitudinal reinforcements14, for example made of Kevlar® or made of carbon, these being placedwithin a plastic matrix. In FIG. 4, the reinforcements incorporated intothe material of the tape 10 a are in the form of a mesh 15, for examplemade of metal. In FIG. 5, the tape 10 a includes longitudinal incisionsor notches 16 that allow passage of the constituent material of thesheath during extrusion of the latter and facilitate the formation oflarger indentations.

1. A flexible pipe comprising, from the inside outward, a carcass formedby a short-pitch helical metal winding wound so as to leave gaps betweenneighboring windings, a covering layer serving as a heat shield, andcomprised of helically wound plastic tapes wound with a winding angle ofless than 35°, an internal sealed pressure sheath comprised of a plasticextruded over the covering layer, at least one armor ply and at leastone external sealing sheath.
 2. The pipe as claimed in claim 1, whereinthe winding angle of the tapes is between 10° and 35°.
 3. The pipe asclaimed in claim 2, wherein the tapes are wound with an overlap.
 4. Thepipe as claimed in claim 3, wherein the tapes have a thickness of lessthan or equal to 1 mm.
 5. The pipe as claimed in claim 4, wherein thecarcass has an outside diameter, and the tapes have a width of about theoutside diameter of the carcass.
 6. The pipe as claimed in claim 5,wherein the covering layer is formed from 3 to 5 tapes.
 7. The pipe asclaimed in claim 4, wherein the tapes are comprised of a reinforcedplastic.
 8. The pipe as claimed in claim 7, wherein the tapes have atensile strength greater than 20 daN.
 9. The pipe as claimed in claim 7,wherein the tapes have longitudinal notches.
 10. The pipe as claimed inclaim 1, wherein the tapes are wound with an overlap.
 11. The pipe asclaimed in claim 1, wherein the tapes have a thickness of less than orequal to 1 mm.
 12. The pipe as claimed in claim 1, wherein the carcasshas an outside diameter, and the tapes have a width of about the outsidediameter of the carcass.
 13. The pipe as claimed in claim 1, wherein thecovering layer is formed from 3 to 5 tapes.
 14. The pipe as claimed inclaim 1, wherein the tapes are comprised of a reinforced plastic. 15.The pipe as claimed in claim 1, wherein the tapes have a tensilestrength greater than 20 daN.
 16. The pipe as claimed in claim 1,wherein the tapes have longitudinal notches.